One possible way of doing this would be to take the file extension of the file and look up the Registry key HKEY_CLASSES_ROOT<file extension>
. There should be a "Content Type" value under this key that gives the MIME type of the file, at least for the most popular file types.
This assumes the file extension is a reliable indicator of the file's contents. On Unix, there is the file command which can look for "magic numbers" inside the file and can return the correct file type even if the extension is wrong. A port of this utility exists for Windows. You might be able to execute it from ASP and capture its output.
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